When two surfaces are brought in contact with each other, a transfer of electrons may occur resulting in a residual static electrical charge when the surfaces are separated. This phenomena is known as triboelectricity. If the surface is composed of a material that is a conductor, the electrons will dissipate quickly thereby eliminating the excess charge. On the other hand, if the surface is composed of a material that is an insulator (a dielectric), the surface charge takes much longer to dissipate. Thermoplastic polymers are typically excellent insulators and thus are unsatisfactory for uses requiring a nature that will dissipate charges. As the polymers accumulate high charges promoting an attraction for dust and dirt, they can discharge to any lower potential body with which they come in contact. To modify a polymer to have antistatic characteristics and dissipate charges, the conductivity might be increased which in turn causes an increase in the rate of static dissipation. This has been accomplished in the past by the use of antistatic agents to promote static-charge decay of surfaces thereby reducing clinging effect, eliminating spark discharge, and preventing accumlation of dust.
It is well known that static charge can be reduced by increasing the moisture content of the atmosphere, and thus the approach in the past has been to use an antistatic agent which will chemically modify the polymer to impart hydrophillic properties to it by providing functional groups that attract moisture to it. For instance, it is well known to apply external antistatic agents onto polymers by conventional coating or painting methods. Also, it is well known to employ internal antistatic agents which are volume dispersed in the polymer; i.e. incorporated into the polymer by compounding or extrusion prior to or during molding or film-forming operations, and work by migrating to the polymer surface. This migration is colloquially referred to in the art of antistatic polymer chemistry as a "blooming" effect. When the antistatic agent has not remained volume dispersed but instead has bloomed to the surface, the mechanism for moisture attraction is the same as with the painted on external antistatic agents. The atmospheric moisture is attracted causing decay or dissipation of static charges, i.e. such films depend on ambient humidity. Accordingly a high rate of blooming is required. Such films can overbloom and lose their antistatic character is subjected to a 24 hour water shower or a prolonged hot, humid atmosphere.
An example of an external antistatic agent is described in U.S. Pat. No. 3,223,545 to Gallaugher et al which discloses a dialkanol amide of the formula R--C(O)--N[(CH.sub.2).sub.n OH].sub.2 wherein R is a C.sub.6 -C.sub.16 alkyl and n is an integer from 2-4, dispersed in a volatile liquid which is applied to the surface of a solid polymer.
Another external antistatic agent applied directly to the surface of a polymeric substrate is described in U.S. Pat. No. 4,268,583 (1981) to Hendy which relates to a film having a polypropylene (PP) substrate and a polymeric heat-sealable surface layer on which is present an antistatic composition comprising (a) a quaternary ammonium compound, such as choline chloride, (b) an organic polyol containing at least two free hydroxyl groups, such as glycerol, (c) a glyceride of a long chain fatty acid, such as glyceryl monostearate, and, optionally, (d) an ethoxylated amine salt, such as an ethoxylated tallow amine sulphate.
Another external antistatic agent is described in U.S. Pat. No. 4,623,594 (1986) to Keough which relates to an antistatic laminate having: (A) a substrate sheet; and (B) a continuous coating on one side of said substrate sheet, said continuous coating being the electron radiation cured product of: (1) an electron beam curable prepolymer; and (2) an effective amount of saturated quaternary ammonium compound antistatic agent soluble in said prepolymer, the product being a reaction product of the prepolymer and the ammonium compound converted into a substantially solid product.
An internal antistatic agent is described in U.S. Pat. No. 3,220,985 to Breslow which discloses modifying hydrocarbon polymers with mono-sulfonazide of the formula RSO.sub.2 N.sub.3, where R is an organic radical inert to the modification reaction, i.e. modifying PP with p-toluene sulfonazide.
Another internal antistatic agent is described in U.S. Pat. No. 3,164,481 to Shibe which discloses combining a quaternary ammonium benzosulfimide with a plastic. Benzofulfimide is also known as saccharin.
Also of interest is the internal antistatic agent described in U.S. Pat. No. 3,576,649 to Brazier. This patent relates to a package for electrically non-conductive pulverulent material. The package has an inner layer of heat sealable ethylene polymer and a fatty acid amide.
Also of interest is the internal antistatic agent described in U.S. Pat. No. 3,441,552 to Rombusch et al. The patent discloses incorporating an alkoxypropylamine of the formula R.sub.1 --O--(CH.sub.2).sub.3 --N(R.sub.2)(R.sub.3) into a polyolefin where R.sub.1 is an alkyl, alkenyl, alkylcycloalkyl, aryl, alkylaryl or alkenylaryl group of 6-25, preferably 8-18 C atoms in the alkyl or alkenyl moieties and 4-18, preferably 6-12 C atoms in the cycloalkyl moiety, and 6-14, preferably 6-10 C atoms in the aryl moiety; R.sub.2 and R.sub.3 can each represent a H atom, or an alkyl or alkenyl group of 1-5 C atoms, i.e., 100 g octadecyloxy-propyl-N,N-dimethylamine blended with 10 kg PP.
Another internal antistatic agent is disclosed in U.S. Pat. No. 4,554,210 (1985) to Long et al, which claims a first and second outer layer of polyethylene having a surface resistivity at least 1.times.10.sup.16 ohms per square; and a middle layer sandwiched therebetween of polyethylene impregnated with a sloughable, electrically-conductive material providing said middle layer with a volume resistivity no more than 1.times.10.sup.3 ohms/cm.
ICI Americas' brochure entitled "Atmer.RTM. 129 Internal Antistatic Agent for Thermoplastic Polymers" advertises using their new glycerol monostearate in PP, low density polyethylene and polyvinylchloride.
Another internal antistatic agent is disclosed in U.S. Pat. No. 4,600,743 (1986) to Shizuki et al which describes an antistatic fiber obtained by melt spinning of a fiber-forming thermoplastic polymer containing at least one of polyoxyalkylene glycol and its derivatives in an amount of not less than 0.5% by weight.
Another internal antistatic agent is disclosed in U.S. Pat. No. 4,117,193 (1978) to Tsuchiya et al, which discloses a film prepared by melt extrusion laminating a polymer blend composition of a low-crystalline resin of an ethylene-butene copolymer and a polyolefin resin having incorporated therein a lubricant and an anti-blocking agent onto surface(s) of a uniaxially stretched PP film followed by stretching the laminate in the direction perpendicular to the direction in which said PP has been stretched and optionally subjecting the resultant to corona discharge.
Also of interest is U.S. Pat. No. 4,605,684 (1986) to Pcolinsky which relates to an internal antistatic agent. It discloses a method of preparing polyurethane foam from a polyol and a polyisocyanate the improvement being adding to the foam-forming composition from about 5 to about 25 parts by weight per 100 parts by weight of polyol of an antistatic composition of one part by weight of a quaternary ammonium compound selected from the group consisting of soya dimethyl ethyl ammonium ethylsulfate, soya dimethyl ethyl ammonium ethylphosphate, and mixtures thereof and from about 0.4 to about 3 parts by weight of plasticizer composition selected from the group consisting of N-ethyl-o- and p-toluene sulfonamide, o- and p-toluene sulfonamide, tetrakis (2 chloroethyl) ethylene diphosphate, and mixtures thereof, to provide a foam having a reduced tendency to develop and accumulate electrostatic charges.
The following patents, which do not relate to antistatic compositions, are also of general interest. For instance, U.S. Pat. No. 4,536,532 (1985) to Miller, relates to a process for the manufacture of a polyvinyl alcohol homopolymer having a vinyl alcohol content in excess of 95% wherein said homopolymer is mixed with a plasticizer selected from the group consisting of N-substituted fatty acid amides; aryl, alkaryl, N-aryl aryl, N-alkaryl aryl and N-alkyl alkaryl sulfonamides and alkaryl sulfonamides; N-alkyl pyrrolidones; sulfonated alkyl phenols; aryl and alkaryl phosphates and phosphites; alkylene carbonates and selected mixtures thereof. Also, a blend of high melting nylon (melting point 415.degree.-440.degree. F.) and ethylene vinyl alcohol copolymer (EVOH) plasticized with lauramide, o,p-toluenesulfonamide, N-ethyl-o,p-toluene-sulfonamide or a polyamide of 7000-10,000 molecular weight, is described in U.S. Pat. No. 4,347,332 to Odorzynski et al.
Polyether block amide copolymers, i.e. polyamide-polyether copolymers (PAEPC), are described in U.S. Pat. No. 4,361,680 (1982) to Borg et al, U.S. Pat. No. 4,332,920 (1982) to Foy et al, and U.S. Pat. No. 4,331,786 (1982) to Foy et al, (all assigned to ATO Chimie), the disclosures of which are incorporated herein by reference. Also an advertising brochure from Atochem, entitled "Pebax.RTM. Technical Notice", June 1986, No. 507E-9E, the disclosure of which is incorporated herein by reference, describes properties of various PAEPC's marketed under the trade-name Pebax.RTM., and in particular at pages 13-15 describes the electrical properties of the various Pebax copolymers. The brochure states that Pebax 4011 by itself is antistatic in nature because it is extremely hydrophillic, as it will absorb 120% of its weight in water when allowed to soak for 24 hours. This material is so hydrophillic that it would be inappropriate for packaging electronic devices as so much moisture from the atmosphere would be attracted that as the water released it would corrode the packaged device. Thus, it is herein defined that by the terms "polyether block amide copolymer", "polyamide-polyether copolymer", and "PAEPC", which are used interchangeably, it is not intended to include the extremely hydrophillic Pebax.RTM. 4011 or any other polyether block amide copolymer that is as hydrophillic as, or substantially as hydrophillic as, Pebax.RTM. 4011. It is intended to include polyether block amide copolymer having a degree of hydrophilicity suitable for packaging electronic devices. While it is not intended to be bound to any theory, it is believed that the extremely hydrophillic characteristic of Pebax 4011 is due to the EP component of the Pebax 4011 copolymer forming a helix with the oxygens inside, which tends to hold moisture inside the helix. Pebax 4011 is about 50% by weight EP and 50% by weight PA. The brochure also states the natural Pebax grades of the Pebax--33 series have a surface resistivity of 10.sup.10 ohms.multidot.cm. The brochure also describes making the Pebax grades of the Pebax--33 series of copolymers semi-conductive by adding carbon black thereto. It is noted here that the addition of carbon black to polymers to make them semi-conductive is old technology and well known to those skilled in the art of antistatic and conductive polymeric chemistry.